Specification of organ/tissue identity is a fundamental requirement of animal development as it is imperative that each tissue/organ type be made in the right numbers, placed in the right location and constructed to function properly. The traditional view of this process has been that gene regulatory networks function exclusively to promote the desired fate. However, a growing body of evidence, including several discoveries from my research group, strongly suggests that determination of organ and tissue identity actually has two requirements: 1) specifying the desired tissue and 2) repressing alternate fates. A situation where this paradigm-shifting model is likely to be particularly valuable is in establishing boundaries between distinct tissue types. In this respect eye development provides an ideal opportunity to test our new models of tissue specification. In Drosophila, several non-ocular structures such as the head epidermis, antenna, and maxillary palp border the developing fly retina. We examined sine oculis (so) and eyes absent (eya) mutant retinas and have determined that selector genes normally expressed in the surrounding non-ocular tissues are ectopically activated in the eye field. Activation of these factors within retinal progenitor clls forces a homeotic transformation of the eye field into epidermal tissue. The vertebrate eye similarly arises from a territory that is bordered by non-ocular tissues including the telencephalon, diencephalon, and hypothalamus. Recent studies have shown that several selector genes controlling their development are also ectopically activated in the eyes of mouse Lhx2 and frog rax mutants. The transformations that are seen in both vertebrates and flies indicate that segregation of ocular and non-ocular fates is essential for proper head and eye formation. The objective of this proposal is to determine how Sine Oculis Homeobox (SIX) and Eyes Absent (EYA) proteins promote eye formation by repressing non-ocular fates in the developing retina. The rationale for the proposed research is that the chosen questions are focused on processes that are likely to be highly conserved, thereby allowing studies in Drosophila to uncover general mechanisms of tissue/organ formation. Exciting preliminary data guides the following specific aims: (1) Investigate the role that SIX/EYA proteins play in the novel suppression of non-ocular fates during eye specification; (2) Test the hypothesis that retinal patterning by the morphogenetic furrow requires suppression of non-ocular fates by Dpp signaling and SIX/EYA proteins; and (3) Identify the molecular mechanism by which eya expression is activated in the eye field and repressed in bordering non-ocular tissues. The proposed studies test innovative hypotheses and are significant because our results will uncover new far-reaching principles governing tissue/organ specification and patterning. The work also will further our understanding of how errors in these processes facilitate the induction of congenital disorders.